internal/types/errors: rename UntypedNil to UntypedNilUse

internal/types/errors: rename UntypedNil to UntypedNilUse

This avoids a conflict when dot-importing this package in
go/types and types2.

Change-Id: Ia6fc45ef21c28ea595b49f5321b5c0d441763e2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/439562
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>